The latest IETF 6TiSCH (IPv6 over Time Synchronized Channel Hopping (TSCH) MAC) standard is considered by many as the de-facto technique for wireless industrial applications, which can offer high communication reliability and low latency by enabling deterministic wireless communication and optimally schedule radio resources among network devices.
In a wireless network, TSCH technology divides the wireless bandwidth into time and frequency and neighbouring wireless network devices (or “nodes”) can communicate (i.e. transmit and/or receive data) during a timeslot over a specific bandwidth (also referred to herein as a frequency channel). A Time Division Multiple Access (TDMA) schedule determines during which timeslot and over which frequency channel a network device should transmit or receive data to/from its neighbours.
The majority of existing scheduling algorithms only address uplink scheduling problems—that is, data to be communicated from distal nodes towards the central hub, or “root node”. However, downlink data—that is, data to be communicated in a direction from the root node towards the distal nodes—is necessary for many industrial applications, for example, sending control messages, querying and reconfiguring network devices and upgrading device firmware. It would therefore be desirable to provide a method and network device for producing schedules which adequately provides for both uplink and downlink data.
Furthermore, many current scheduling algorithms are suitable only for use with small scale networks. This is due to the overflow of a node buffer—that is a exceeding of a limitation on the amount of data a single network device can store. This is often caused by nodes closer to the central hub, or root node, needing to relay data from more distal nodes. This means that the more central nodes need to receive and store data from other nodes, before transmitting it on to the next node. Many scheduling algorithms result in a centrally-located node receiving a large amount of data before being able to transmit it on and this can overflow the buffer, resulting in lost data or malfunction of the wireless local area network. It would therefore also be desirable to provide a method which minimises the amount of data which needs to be stored at individual nodes at any one time.
Existing scheduling algorithms will typically be undertaken centrally. A centralised scheduler will require data communication requirements from all of the nodes in a network and will then prepare a schedule for transmission of all of the network data, attempting to avoid collisions and interference. This process can be complex and time consuming. Distributed scheduling schemes alleviate the problems associated with centralised systems, by making local and thus quicker scheduling decisions. Known distributed scheduling schemes build a schedule in a sequential manner starting from the most distal nodes and working towards the root node. Such schemes often take a long time to start-up when the network size is large. A scheduler is sought which can overcome these disadvantages.